(a) Field of the invention:
The present invention relates to a surface structure measuring apparatus.
(b) Description of the prior art:
Of late, non-contacting optical measuring systems have become employed for the measurement of surface structures. Among various optical section curve measuring systems those representing applications of the focus detecting method are attracting the attention of those concerned as having the possibility of elevating the degree of precision of measurement and of making the apparatus compact. Of these systems, there are those which utilize the focus detecting method such as the critical angle technique, the astigmatism technique, the pupil dividing technique, etc. as the techniques which are presently proposed
Description will be made first of the system which utilizes the critical angle technique. FIG. 1 is an illustration showing the principle of the critical angle technique. In case the measurement surface is located at a focal position 2 of an objective lens 1, the light 5 which is reflected at the measurement surface is rendered to a parallel light 6 by the objective lens 1 and enters into a critical angle prism 7. By setting the prism 7 so as to perform total reflection of the incident light at such a time, a same amount of light arrives at two photodiodes 8 and 9, respectively. Also, in case the measurement surface is located at a position 3 which is closer to the objective lens 1 rather than the focal position is close to the objective lens 1, the reflected light, after passing through the objective lens 1, will become a diverging light 10 and will enter into the critical prism 7. At such a time, there arises a difference in the incidence angle of light on one side of the optical axis relative to the other side thereof. Therefore, the light located on that side which does not satisfy the total reflection conditions will travel to the outside of the prism 7 as being a light 11, whereas the light located on that side of the optical axis which satisfies the total reflection conditions is reflected totally and arrives mainly at a photo-diode 9. Accordingly, at such a time, only a small amount of light arrives at the photo-diode 8. Also, in case the measurement surface is located at a position 4 which is farther from the objective lens 1 rather than the focal position 2 is far from the objective lens 1, the situation is just the reverse of the instance wherein the measurement surface is located at the position 3, and only a small amount of light will arrive at the photo-diode 9. Accordingly, by reading out the outputs of these two photo-diodes 8 and 9 by means of an operation amplifier 12, there is obtained a displacement-to-output characteristic as shown in FIG. 2.
FIG. 3 shows an optical system of Model No. HIPOSET 10 (tradename) made by Kabushiki Kaisha Kosaka Kenkysho as an example of the prior art surface structure measuring apparatus utilizing the abovesaid critical angle technique. The infrared laser beam emitting from a laser diode 13 passes through a collimator lens 14, a deflected light beam splitter 15, a .lambda./4 plate 16 and an objective lens 17, and impinges onto a sample 18. The reflected light beam, in turn, passes through the objective lens 17, the .lambda./4 plate 16, the deflected light beam splitter 15 and a beam splitter 19, and impinges onto a critical angle prism 20 or 21. Two pairs of photo-diodes 22, 23 and 24, 25 are provided on the rear side of the critical angle prisms 20 and 21, respectively. From their respective outputs, there is detected the position of the sample 18 in the optical axial direction. And, by performing the scanning the surface of the sample by moving the sample 18 mechanically, its surface structure can be detected.
FIGS. 4A, 4B and 4C are illustrations showing the principle of the astigmatism technique. This is a technique arranged so that a cylindrical lens is disposed rearwardly of an objective lens not shown to impart astigmatism to the optical system so that the variation of the sectional structure of the bundle of light caused by the aberration or shifting of the measurement surface from the focal position is caught by a detector. More particularly, in case the optical system contains astigmatism, it will be noted that, when a spot image coming from a spot light source such as a laser impinges, the shape of this spot image will vary as from the shape 26 onto the shape 27 and to the shape 28 in the foreground of the focal position, in the focal position and in the background thereof relative to those as shown in FIGS. 4A, 4B and 4C, respectively. By detecting these images with quadrant detectors 29, 30, 31 and 32 and by subjecting them to arithmetic operation: (V.sub.29 +V.sub.31)-(V.sub.30 +V.sub.32), there is obtained an output-to-displacement characteristic similar to that of FIG. 2, and thus the focal point can be detected. Here, the values of respective V's represent the outputs of the detectors assigned with corresponding reference numerals.
FIG. 5A shows the optical system of an example of the surface structure measuring apparatus utilizing the abovesaid principle of astigmatism. The laser light beam emitting from a laser light source 33 passes through a spatial filter 34 and impinges onto a deflected light beam splitter 35, and after passing through a .lambda./4 plate 36 and through an objective lens 37, it impinges onto a sample 38. The reflected light passes through the objective lens 37, the .lambda./4 plate 36, the deflected light beam splitter 35 and a beam splitter 39, and impinges onto cylindrical lenses 40 and 41. These cylindrical lenses 40 and 41 are provided there to develop astigmatism, respectively. In the background of these cylindrical lenses 40 and 41, there are provided detectors 42 and 43, respectively. These detectors are each comprised of four photo-diodes 44, 45, 46 and 47, respectively, as shown in FIG. 5B, and they detect the focal position in accordance with the principle which has been described in connection with FIGS. 4A, 4B and 4C. And, by mechanically moving the sample in the arrow direction, amounts of defocus corresponding to the surface structures of the sample thus moved are detected by these detectors 42 and 43. By converting these outputs to the amounts of displacement in accordance with the relationship mentioned in FIG. 2, the surface structure can be measured. Focusing is obtained by driving either a stage or the objective lens by a feedback circuit using the focal position detection signal of said device as an error signal. In the surface structure measuring method, however, arrangement is provided so that focal position detection signal itself is grasped as the variation of the surface structure, and measurement is made based thereon.
FIGS. 6A, 6B and 6C are illustrations of an image showing the principle of the pupil dividing technique. FIG. 6B shows the instance wherein focus is obtained. The light coming from the object spot 51 passes through an objective lens 48, and thereafter the bundle of light is reduced to one half by a light interrupting plate 49 which shuts one half of the pupil of said lens. When focus is obtained, an image is produced in the center of a detector 50. FIG. 6A shows the instance wherein the object spot 52 is displaced closer toward the objective lens 48 from the focal position. An image is produced at a position located upper than the center of the detector 50 by virtue of the light bundle which has been reduced to one half by the light interrupting plate 49. FIG. 6C shows the instance wherein the object spot 53 is displaced from the focal position to a position farther away from the objective lens 48. In this instance, an image is produced at a position below the center of the detector 50 by virtue of the bundle of light which has been reduced to one half by the light interrupting plate 49. By blocking one half of the pupil in such a manner as stated above, it is possible to convert the displacement of the focal position to the shifting of the position of the image. Accordingly, by constructing the detector 50 either by a partial detector or by a position detector, the detection of focal position becomes feasible. This principle is disclosed in the Japanese Patent Preliminary Publication No. Sho 58-194007.
FIG. 7 shows the optical system of an example of the surface structure measuring apparatus utilizing the principle of said pupil dividing technique. The laser light beam emitting from a laser light beam source 54 passes through a spatial filter 55, a collimator lens 56, a deflected light beam splitter 57, a .lambda./4 plate 58 and an objective lens 59, and impinges onto a sample 60. The light reflected at the sample 60 passes through the objective lens 59, the .lambda./4 plate 58 and the deflected light beam splitter 57, and impinges onto a pupil dividing mirror 61. The bundle of light which has lost its one half by the pupil dividing mirror 61 is caused by a focusing lens 62 to impinge onto photo-diodes 63 and 64. Also, the bundle of light which is reflected by only one half of the original bundle of light by the pupil dividing mirror 61 is caused by a focusing lens to impinge onto photo-diodes 66 and 67. Accordingly, by carrying out an arithmetic operation: (V.sub.63 -V.sub.64)+(V.sub.66 -V.sub.67), it is possible to detect the focal position by relying on the principle described in connection with FIGS. 6A, 6B and 6C. And, when the sample 60 is moved mechanically in the arrow direction, the focus detection signal varies. However, its values are to be noted to indicate the surface structure by giving reference to the relationship shown in FIG. 2
Here, the respective outputs V.sub.63, V.sub.64 ; V.sub.66, V.sub.67 which are produced by the respective paired photo-diodes 63, 64; 66, 67 vary by virtue of the variation of the rate of reflection at the measurement surface. However, by normalizing them by the sum of the signals as by: ##EQU1## it is possible to compensate for the fluctuation of the amount of light caused by the variation of the rate of reflection. Also, instead of the halved photo-diodes, there may be employed, for example, a semiconductor position detector. It should be noted here also that two pairs of detecting optical systems are provided in FIGS. 3, 5A, 5B and 7 to cancel errors caused by an inclination of the sample.
However, the above-described techniques of the prior art invariably use a laser beam only on the axis of the optical system, and scanning is performed by mechanically shifting the sample. As a result, there have been the drawbacks such that the measurement speed is slow, that the shape, the weight, etc. of the sample are limited, and further that the measurement surface cannot be observed